You've heard of the "coffin corner?"
The early Lears, at max ceiling, were only 3-5 knots from stall on the low end and 3-5 knots from Mach tuck if they went any faster.
In the P-47, as in any normal aircraft, the stall speed goes up with decreasing air density and it is probable that if the service ceiling was 43,000 feet, then the stall margin at 30,000 feet was smaller than appreciated. He may only have had 3 g available or perhpas even less, so a mild pull on the stick at sea level could easily tunr ino a stall at high altitude. Nobody was dogfighting at 35,000 feet in WWII ... they were making gentle turns and trying to get a good sight picture.
I have read about encounters between Grumman Panthers and Soviet MiGs at high altitude and all it took to spoil the MiG's aim was a slight turn into the path of the MiG ... and there was simply not eough g-available for the MiG to get a firing solution, so he'd go around again and make another try. They played cat and mouse for several passes and the MiGs got low on fuel and departed for home base as the Panther withdrew.